This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Inorganic mercury results toxic because it reacts with the sulfur atoms in proteins interfering with their physiological function. Bacteria avoid such toxicity by transporting the mercury found in the periplasm to the cytoplasm where the toxic Hg2+ ion is reduced to the less toxic Hg0 form. Three proteins in the bacterial mer operon are directly involved in the transport mechanism: a periplasmic mercury binding protein (MerP), a transporter (MerT) and a mercury reductase (MerA). Our specific goal is to understand at atomic level how periplasmic mercury is transported across the membrane to the cytoplasm to reach the mercury reductase active site without first reacting with other proteins in the cell. Cysteine residues in the three proteins are known to be involved in the transport mechanism, but a defined pathway for the mercuric ion is yet to be determined.